The lake I fish has water visibility to about 10 ft,after that depth the water is a green color(surface). The lake has a maximum depth of 75-80 ft.

My wife was asking my why the water has a green appearance to it,even out in the middle where the water is 60+ feet deep.

I dont have a good answer,hope someone could help me with this question.

Because it contains algae which reflects the blue/green light of the spectrum.

Algae and or bottom composition. The system I normaly fish has a mixture of gin clear water and stained water in the early spring and late fall. During the summer the entire system is stained from the algae. Also the excessive boat traffic stirs things up contributing to the staining.

What you are seeing is phytoplankton -minute suspended algae that forms some fraction of the base of the food chains in your lake. There are different species that grow at different times of the year, but most grow in summer, esp after sunny periods. Such rapid growth after sunny periods is called a "bloom".